Page No 224:

Question 1:

Why does a compass needle get deflected when brought near a bar
magnet?

Answer:

A compass
needle is a small bar magnet. When it is brought near a bar magnet,
its magnetic field lines interact with that of the bar magnet. Hence,
a compass needle shows a deflection when brought near the bar magnet.

Page No 228:

Question 1:

Draw
magnetic field lines around a bar magnet.

Answer:

Magnetic
field lines of a bar magnet emerge from the north pole and terminate
at the south pole. Inside the magnet, the field lines emerge from the
south pole and terminate at the north pole, as shown in the given
figure.

Page No 228:

Question 2:

List the
properties of magnetic lines of force.

Answer:

The
properties of magnetic lines of force are as follows.

(a) Magnetic
field lines emerge from the north pole.

(b) They
merge at the south pole.

(c) The direction of field lines inside the magnet is from the south
pole to the north pole.

(d) Magnetic
lines do not intersect with each other.

Page No 228:

Question 3:

Why don’t two magnetic lines of force intersect each other?

Answer:

If two
field lines of a magnet intersect, then at the point of intersection,
the compass needle points in two different directions. This is not
possible. Hence, two field lines do not intersect each other.

Page No 229:

Question 1:

Consider a
circular loop of wire lying in the plane of the table. Let the
current pass through the loop clockwise. Apply the right-hand rule to
find out the direction of the magnetic field inside and outside the
loop.

Answer:

Inside the
loop = Pierce inside the table

Outside
the loop = Appear to emerge out from the table

For
downward direction of current flowing in the circular loop, the
direction of magnetic field lines will be as if they are emerging
from the table outside the loop and merging in the table inside the
loop. Similarly, for upward direction of current flowing in the
circular loop, the direction of magnetic field lines will be as if
they are emerging from the table outside the loop and merging in the
table inside the loop, as shown in the given figure.

Page No 229:

Question 2:

The magnetic field in a given region is uniform. Draw a diagram to
represent it.

Answer:

The
magnetic field lines inside a current-carrying long straight solenoid
are uniform.

Page No 230:

Question 3:

Choose the
correct option.

The
magnetic field inside a long straight solenoid-carrying current

(a) is
zero

(b) decreases
as we move towards its end

(c) increases
as we move towards its end

(d) is the
same at all points

Answer:

(d)The magnetic field inside a long, straight,
current-carrying solenoid is uniform. It is the same at all points
inside the solenoid.

Page No 231:

Question 1:

Which of
the following property of a proton can change while it moves freely
in a magnetic field? (There may be more than one correct answer.)

(a) mass

(b) speed

(c) velocity

(d) momentum

Answer:

(c) and
(d)

When a
proton enters in a region of magnetic field, it experiences a
magnetic force. As a result of the force, the path of the proton
becomes circular. Hence, its velocity and momentum change.

Page No 232:

Question 2:

In
Activity 13.7 (page: 230), how do we think the displacement of rod AB
will be affected if (i) current in rod AB is increased: (ii) a
stronger horse-shoe magnet is used: and (iii) length of the rod AB is
increased?

Answer:

A
current-carrying conductor placed in a magnetic field experiences a
force. The magnitude of force increases with the amount of current,
strength of the magnetic field, and the length of the conductor.
Hence, the magnetic force exerted on rod AB and its deflection will
increase if

(i) current
in rod AB is increased

(ii) a
stronger horse-shoe magnet is used

(iii) length
of rod AB is increased

Page No 232:

Question 3:

A
positively-charged particle (alpha-particle) projected towards west
is deflected towards north by a magnetic field. The direction of
magnetic field is

(a) towards south

(b) towards east

(c) downward

(d) upward

Answer:

(d) The direction of the magnetic field can be determined by the
Fleming’s left hand rule. According this rule, if we arrange
the thumb, the centre finger, and the forefinger of the left hand at
right angles to each other, then the thumb points towards the
direction of the magnetic force, the centre finger gives the
direction of current, and the forefinger points in the direction of
magnetic field. Since the direction of positively charged alpha
particle is towards west, the direction of current will be the same
i.e., towards west. Again, the direction of magnetic force is towards
north. Hence, according to Fleming’s left hand rule, the
direction of magnetic field will be upwards.

Page No 233:

Question 1:

State
Fleming’s left-hand rule.

Answer:

Fleming’s
left hand rule states that if we arrange the thumb, the centre
finger, and the forefinger of the left hand at right angles to each
other, then the thumb points towards the direction of the magnetic
force, the centre finger gives the direction of current, and the
forefinger points in the direction of magnetic field.

Page No 233:

Question 2:

What is the principle of an electric motor?

Answer:

The
working principle of an electric motor is based on the magnetic
effect of current. A current-carrying loop experiences a force and
rotates when placed in a magnetic field. The direction of rotation of
the loop is given by the Fleming’s left-hand rule.

Page No 233:

Question 3:

What is
the role of the split ring in an electric motor?

Answer:

The split
ring in the electric motor acts as a commutator. The commutator
reverses the direction of current flowing through the coil after each
half rotation of the coil. Due to this reversal of the current, the
coil continues to rotate in the same direction.

Page No 236:

Question 1:

Explain
different ways to induce current in a coil.

Answer:

The
different ways to induce current in a coil are as follows:

(a) If a coil is moved rapidly between the two poles of a horse-shoe
magnet, then an electric current is induced in the coil.

(b) If a magnet is moved relative to a coil, then an electric current
is induced in the coil.

Page No 237:

Question 1:

State the
principle of an electric generator.

Answer:

An
electric generator works on the principle of electromagnetic
induction. It generates electricity by rotating a coil in a magnetic
field.

Page No 237:

Question 2:

Name some
sources of direct current.

Answer:

Some
sources of direct current are cell, DC generator, etc.

Page No 237:

Question 3:

Which
sources produce alternating current?

Answer:

AC generators, power plants, etc., produce alternating current.

Page No 237:

Question 4:

Choose the correct option.

A
rectangular coil of copper wires is rotated in a magnetic field. The
direction of the induced current changes once in each

(a) two revolutions

(b) one revolution

(c) half revolution

(d) one-fourth revolution

Answer:

(c) When a rectangular coil of copper is rotated in a magnetic field,
the direction of the induced current in the coil changes once in each
half revolution. As a result, the direction of current in the coil
remains the same.

Page No 238:

Question 1:

Name two safety measures commonly used in electric circuits and
appliances.

Answer:

Two safety
measures commonly used in electric circuits and appliances are as
follows:

(i) Each circuit must be connected with an electric fuse. This
prevents the flow of excessive current through the circuit. When the
current passing through the wire exceeds the maximum limit of the
fuse element, the fuse melts to stop the flow of current through that
circuit, hence protecting the appliances connected to the circuit.

(ii) Earthing is a must to prevent electric shocks. Any leakage of
current in an electric appliance is transferred to the ground and
people using the appliance do not get the shock.

Page No 238:

Question 2:

An
electric oven of 2 kW is operated in a domestic electric circuit (220
V) that has a current rating of 5 A. What result do you expect?
Explain.

Answer:

Current
drawn by the electric oven can be obtained by the expression,

P = VI

Where,

Current =
I

Power of
the oven, P = 2 kW = 2000 W

Voltage
supplied, V = 220 V

Hence, the
current drawn by the electric oven is 9.09 A, which exceeds the safe
limit of the circuit. Fuse element of the electric fuse will melt and
break the circuit.

Page No 238:

Question 3:

What
precaution should be taken to avoid the overloading of domestic
electric circuits?

Answer:

The
precautions that should be taken to avoid the overloading of domestic
circuits are as follows:

(a) Too
many appliances should not be connected to a single socket.

(b) Too
many appliances should not be used at the same time.

(c) Faulty
appliances should not be connected in the circuit.

(d) Fuse
should be connected in the circuit.

Page No 240:

Question 1:

Which of
the following correctly describes the magnetic field near a long
straight wire?

(a) The
field consists of straight lines perpendicular to the wire

(b) The
field consists of straight lines parallel to the wire

(c) The
field consists of radial lines originating from the wire

(d) The
field consists of concentric circles centred on the wire

Answer:

(d) The magnetic field lines, produced around a straight
current-carrying conductor, are concentric circles. Their centres lie
on the wire.

Page No 240:

Question 2:

The
phenomenon of electromagnetic induction is

(a) the
process of charging a body

(b) the process of generating magnetic field due to a current passing
through a coil

(c) producing induced current in a coil due to relative motion
between a magnet and the coil

(d) the
process of rotating a coil of an electric motor

Answer:

(c) When a straight coil and a magnet are moved relative to each
other, a current is induced in the coil. This phenomenon is known as
electromagnetic induction.

Page No 240:

Question 7:

Answer:

Page No 241:

Question 8:

How does a
solenoid behave like a magnet? Can you determine the north and south
poles of a current-carrying solenoid with the help of a bar magnet?
Explain.

Answer:

A solenoid
is a long coil of circular loops of insulated copper wire. Magnetic
field lines are produced around the solenoid when a current is
allowed to flow through it. The magnetic field produced by it is
similar to the magnetic field of a bar magnet. The field lines
produced in a current-carrying solenoid is shown in the following
figure.

In the
above figure, when the north pole of a bar magnet is brought near the
end connected to the negative terminal of the battery, the solenoid
repels the bar magnet. Since like poles repel each other, the end
connected to the negative terminal of the battery behaves as the
north pole of the solenoid and the other end behaves as a south pole.
Hence, one end of the solenoid behaves as a north pole and the other
end behaves as a south pole.

Page No 241:

Question 9:

When is
the force experienced by a current-carrying conductor placed in a
magnetic field largest?

Answer:

The force
experienced by a current-currying conductor is the maximum when the
direction of current is perpendicular to the direction of the
magnetic field.

Page No 241:

Question 10:

Imagine
that you are sitting in a chamber with your back to one wall. An
electron beam, moving horizontally from back wall towards the front
wall, is deflected by a strong magnetic field to your right side.
What is the direction of magnetic field?

Answer:

The
direction of magnetic field is given by Fleming’s left hand
rule. Magnetic field inside the chamber will be perpendicular to the
direction of current (opposite to the direction of electron) and
direction of deflection/force i.e., either upward or downward. The
direction of current is from the front wall to the back wall because
negatively charged electrons are moving from back wall to the front
wall. The direction of magnetic force is rightward. Hence, using
Fleming’s left hand rule, it can be concluded that the
direction of magnetic field inside the chamber is downward.

Page No 241:

Question 11:

Draw a
labelled diagram of an electric motor. Explain its principle and
working. What is the function of a split ring in an electric motor?

Answer:

An
electric motor converts electrical energy into mechanical energy.

It works
on the principle of the magnetic effect of current. A
current-carrying coil rotates in a magnetic field. The following
figure shows a simple electric motor.

When a
current is allowed to flow through the coil MNST by closing the
switch, the coil starts rotating anti-clockwise. This happens because
a downward force acts on length MN and at the same time, an upward
force acts on length ST. As a result, the coil rotates
anti-clockwise.

Current in
the length MN flows from M to N and the magnetic field acts from left
to right, normal to length MN. Therefore, according to Fleming’s
left hand rule, a downward force acts on the length MN. Similarly,
current in the length ST flows from S to T and the magnetic field
acts from left to right, normal to the flow of current. Therefore, an
upward force acts on the length ST. These two forces cause the coil
to rotate anti-clockwise.

After half
a rotation, the position of MN and ST interchange. The half-ring D
comes in contact with brush A and half-ring C comes in
contact with brush B. Hence, the direction of current in the
coil MNST gets reversed.

The
current flows through the coil in the direction TSNM. The reversal of
current through the coil MNST repeats after each half rotation. As a
result, the coil rotates unidirectional. The split rings help to
reverse the direction of current in the circuit. These are called the
commutator.

Page No 241:

Question 12:

Name some
devices in which electric motors are used?

Answer:

Some
devices in which electric motors are used are as follows:

(a) Water
pumps

(b) Electric
fans

(c) Electric
mixers

(d) Washing
machines

Page No 241:

Question 13:

A coil of
insulated copper wire is connected to a galvanometer. What will
happen if a bar magnet is (i) pushed into the coil, (ii) withdrawn
from inside the coil, (iii) held stationary inside the coil?

Answer:

A current
induces in a solenoid if a bar magnet is moved relative to it. This
is the principle of electromagnetic induction.

(i) When a bar magnet is pushed into a coil of insulated copper wire,
a current is induced momentarily in the coil. As a result, the needle
of the galvanometer deflects momentarily in a particular direction.

(ii) When the bar magnet is withdrawn from inside the coil of the
insulated copper wire, a current is again induced momentarily in the
coil in the opposite direction. As a result, the needle of the
galvanometer deflects momentarily in the opposite direction.

(iii) When a bar magnet is held stationary inside the coil, no
current will be induced in the coil. Hence, galvanometer will show no
deflection.

Page No 241:

Question 14:

Two
circular coils A and B are placed closed to each other. If the
current in the coil A is changed, will some current be induced in the
coil B? Give reason.

Answer:

Two
circular coils A and B are placed close to each other.
When the current in coil A is changed, the magnetic field
associated with it also changes. As a result, the magnetic field
around coil B also changes. This change in magnetic field
lines around coil B induces an electric current in it. This is
called electromagnetic induction.

Page No 241:

Question 15:

State the
rule to determine the direction of a (i) magnetic field produced
around a straight conductor-carrying current, (ii) force experienced
by a current-carrying straight conductor placed in a magnetic field
which is perpendicular to it, and (iii) current induced in a coil due
to its rotation in a magnetic field.

Answer:

(i) Maxwell’s right hand thumb rule

(ii) Fleming’s
left hand rule

(iii) Fleming’s
right hand rule

Page No 241:

Question 16:

Explain
the underlying principle and working of an electric generator by
drawing a labelled diagram. What is the function of brushes?

Answer:

The
principle of working of an electric generator is that when a loop is
moved in a magnetic field, an electric current is induced in the
coil. It generates electricity by rotating a coil in a magnetic
field. The following figure shows a simple AC generator.

MNST →
Rectangular coil

A
and B → Brushes

C
and D → Two slip
rings

X →
Axle, G →
Galvanometer

If axle Xis rotated clockwise, then the length MN moves upwards while
length ST moves downwards. Since the lengths MN and ST are moving in
a magnetic field, a current will be induced in both of them due to
electromagnetic induction. Length MN is moving upwards and the
magnetic field acts from left to right. Hence, according to Fleming’s
right hand rule, the direction of induced current will be from M to
N. Similarly, the direction of induced current in the length ST will
be from S to T.

The
direction of current in the coil is MNST. Hence, the galvanometer
shows a deflection in a particular direction. After half a rotation,
length MN starts moving down whereas length ST starts moving upward.
The direction of the induced current in the coil gets reversed as
TSNM. As the direction of current gets reversed after each half
rotation, the produced current is called an alternating current (AC).

To get a
unidirectional current, instead of two slip rings, two split rings
are used, as shown in the following figure.

In this
arrangement, brush A always remains in contact with the length
of the coil that is moving up whereas brush B always remains
in contact with the length that is moving down. The split rings C
and D act as a commutator.

The
direction of current induced in the coil will be MNST for the first
rotation and TSNM in the second half of the rotation. Hence, a
unidirectional current is produced from the generator called DC
generator. The current is called AC current.

Page No 241:

Question 17:

When does an electric short circuit occur?

Answer:

If the
resistance of an electric circuit becomes very low, then the current
flowing through the circuit becomes very high. This is caused by
connecting too many appliances to a single socket or connecting high
power rating appliances to the light circuits. This results in a
short circuit.

When the
insulation of live and neutral wires undergoes wear and tear and then
touches each other, the current flowing in the circuit increases
abruptly. Hence, a short circuit occurs.

Page No 241:

Question 18:

What is
the function of an earth wire? Why is it necessary to earth metallic
appliances?

Answer:

The
metallic body of electric appliances is connected to the earth by
means of earth wire so that any leakage of electric current is
transferred to the ground. This prevents any electric shock to the
user. That is why earthing of the electrical appliances is necessary.